Electric valve circuits



May 29, 1934. O UvlNGSTON 1,961,057

ELECTR 10 VALVE G IRCUITS Filed Dec. 16. 1931 II I! Fig.2.

I I I Inventor: Orrin W. Livingston,

b W M His Attorneg.

Patented May 29, 1934 UNITED STATES PATENT OFFICE ELECTRIC VALVECIRCUITS New York Application December 16, 1931, Serial No. 581,396

4 Claims.

My invention relates to electric valve circuits and more particularly tosuch circuits including valves of the vapor electric discharge type.

Heretofore, there have been devised numerous apparatus includingelectric valves for transmitting energy between direct current circuits,direct and alternating current circuits, or alternating current circuitsof different frequencies, voltages, phases, etc. In such electric valveconverting apparatus, it has been found particularly advantageous to usevalves of the vapor electric discharge type because of the relativelylarge amounts of power which may be handled at ordinary operatingvoltages. In the majority of the arrangements of the prior art, however,the several electric valves are adapted to be rendered conductiveperiodically for short intervals of time and to be maintainednon-conductive periodically for other intervals of time. In suchapparatus it has been customary to maintain the valves non-conductiveduring the desired intervals by impressing a negative potential upon thecontrol grid, this potential being either a periodic potential, eachcycle of which has an appropriate negative period, or a unidirectionalnegative bias potential upon which is superimposed a periodic potential,the positive periods of which are sufficient to render the gridspositive during intervals in which the valve is to be made conductive.

In many of the electric valve converting apparatus, it has beenfound'that periodic transients of very steep wave front are set up inthe apparatus due to the sudden transfer of load current between theseveral electric valves. In case these transients are impressed across anon-conducting electric valve with such a polarity that the anode ismade positive with respect to the cathode, it has been found that theinternal capacity between the grid and anode of the electric valve isoften sufficient to permit a potential to be electrostatically inducedupon the grid of such a magnitude as to overcome the grid bias andrender the valve conducting; that is, the grid of the electric valveloses control or" its conductivity. Such an occurrence usually causes ashort circuit upon one or more of the parts of the apparatus withconsequent damage thereto and interruption of service.

It is an object of my invention, therefore, to provide an improvedelectric valve circuit for valves of the vapor electric discharge typewhich will overcome the above mentioned disadvantages of thearrangements of the prior art and which will be simple and reliable inoperation.

It isa'nother object of my invention to provide .electric valves 14 and15.

an improved electric valve circuit including a valve of the vaporelectric discharge type which will avoid the possibility of the gridlosing control of the conductivity of the valve upon the occurrence oftransients in the valve circuit.

In accordance with my invention, the effect of the transient positivevoltages in the anode circuit of a vapor electric discharge valve on thegrid of the device, tending to deprive the grid of control over theconductivity of the valve, is substantially eliminated by connecting aninductance in the anode circuit of the valve immediately adjacent thevalve. An inductance so connected serves to suppress potentialtransients of steep wave front from the anode of the valve. Thisinductance should preferably be so small as not to affect appreciablythe normal operation of the apparatus, but, due to the steepness of thewave front of the disturbing transients, its reactance to thesetransients will be sufiicient to suppress them substantially from theanode of the valve.

For a better understanding of my invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims. Referring now to the drawing, thereis illustrated in Fig. 1, an electric valve converting apparatus of thetype known in the art as a parallel inverter in which my invention isembodied, while Fig. 2 is a copy of an oscillogram illustratingcertain-operating characteristics of the apparatus of Fig. 1. v

In Fig. 1 of the'drawing there is illustrated an electric valveconverting apparatus embodying my invention adapted to transmit energyfrom a direct current supply circuit 10 to an alternating current loadcircuit 11. This apparatus comprises a transformer 12 having a secondarywinding connected to the alternating current 11 and a primary windingprovided with an electrical midpoint connected to one side of the directcurrent circuit 10 through a smoothing reactor 13 and a pair of endterminals connected to the other side of the direct current circuit 10through vapor electric discharge valves 14 and 15. A commutatingcapacitor 16 is connected between The vapor electric discharge valves 14and 15 are each provided with an anode, a cathode and a control grid forcontrolling the starting of current in the anode cathode circuit.

The grids of the valves 14 and 15 are connected to their common cathodecircuit through opposite halves of the secondary winding of a gridtransformer 1'7 and a current limiting resistor 18. The primary windingof the grid transformer 17 may be energized from any suitablealternating current circuit 19 of the frequency which it is desired tosupply to the circuit 11, or, in case the circuit 11 is connected to anindependent source of electromotive force for determining its frequency,the primary winding of the transformer 17 may be energized therefromthrough any suitable phase advancing condenser, as will be wellunderstood by those skilled in the art.

In order substantially to eliminate the effect of transients in theanode circuits of the electric valves upon the valves, there areincluded in the anode circuits of the valves 14 and 15 reactors 20 and21, respectively, each being connected immediately adjacent the anode ofits respective valve. These inductances are preferably of such a lowvalue that they have no appreciable effect upon the operation of theapparatus at its normal frequency. However, due to the very steep wavefront of the transients in the anode circuits these inductances presenta very high reactance and substantially suppress these disturbingtransients from the anodes of the electric valves. The inductances 20and 21 should preferably have a very low distributed capacity in orderto obtain their maximum effectiveness. If desired, capacitors 22 and 23may be connected between the grids and cathodes of the valves 14 and 15,respectively, although this feature of connecting a capacitor betweenthe grid and cathode of a vapor electric discharge valve to eliminatethe effects of transients in the anode circuit forms no part of mypresent invention, but is disclosed and claimed in a copendingapplication of Alan Howard, Serial No. 566,376, filed October 1, 1931,and assigned to the same assignee as the present application.

The general principles of operation of the above described apparatuswill be well understood by those skilled in the art or may be foundexplained in detail in United States Letters Patent No. 1,800,002,granted April 7, 1931, upon the application of E. F. W. Alexanderson. Inbrief, if one of the electric valves, for example valve 14, is initiallymade conductive by a positive half cycle of grid potential from thetransformer 17, current will flow from the positive side of the directcurrent circuit through the left hand portion of the primary winding ofthe transformer 12 and electric valve 14 to the other side of the directcurrent circuit. During this interval, capacitor 16 will become chargedto a potential substantially twice that of the direct current circuitand, when electric valve 15 is made conductive, the potential of thiscapacitor is effective to transfer the current from the valve 14 to thevalve 15. The current will now flow through the right hand portion ofthe primary winding of the transformer 12 and electric valve 15 to theother side of the direct current circuit. In this manner current issuccessively commutated between electric valves 14 and 15 flowing inopposite directions through the primary winding of transformer 12 andsupplying an alternating potential to the circuit 11. In case thealternating current circuit 11 is connected to an independent source ofelectromotive force, it will be understood that the capacitor 16 may beomitted and the primary winding of the grid transformer may be energizedfrom the alternating current circuit 11, in which case the electromotiveforce of the transformer 12 is effective to commutate the currentbetween the electric valves.

In Fig. 2 there is illustrated a copy of an oscillogram of the potentialappearing across one of the electric valves in a circuit of the typeillustrated in Fig. 1. The portions (1 of this curve represent intervalsduring which the valve is conducting current so that the potentialacross the valve is only the arc drop of the valve, which remainssubstantially constant. The intervals I) of this curve represent theperiods during which the valve is non-conductive. It will be seen that,diu'ing the initial portions of these intervals b, the anode potentialof the valve rises very rapidly to a high positive value. As explainedin more detail in the copending application of Alan Howard, Serial No.566,376, referred to above, the capacity between the grid and anode ofeach of the valves 14 and 15 causes a potential to be electrostaticallyinduced upon the control grid, the magnitude of which depends upon theinter-electrode capacities of the valves. By means of the capacitors 22and 23, connected between the grids and cathode of the valves, the valueof the potential electrostatically induced upon the control grids may bereduced substantially. However, I have found that the reliability of thegrid control circuits may be substantially improved by means of theinductances 20 and 21 connected in the anode circuit, these inductancestending to absorb the potential transients of steep wave front byoffering a very high impedance to the charging currents for theinter-electrode capacities of the valves and the additional capacitorsconnected between the grids and cathodes of the valves. That is, duringthe intervals 17 of the curve of Fig. 2, when a positive potentialtransient appears in the anode circuit of the electric valve, the dangerof the valve being rendered conductive due to the grid being deprived ofits control by potentials electrostatically induced on the grid, issubstantially eliminated.

While I have described what I at present consider the preferredembodiment of my invention, it will be obvious to those skilled in theart that various changes and modifications may be made without departingfrom my invention, and I, therefore, aim in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination with an electric power con verting apparatus includinga plurality of vapor electric discharge valves, each provided with ananode, a cathode and a control grid, the grid and anode of each of saidvalves forming a substantial internal capacity, means for rapidlytransferring the load current of the apparatus between said electricvalves, the anode circuit of certain of said valves being subject totransient potential waves of steep wave front upon each sudden transferof current between said valves, and means for substantially eliminatingthe effeet on said control grids of said transients in the anodecircuits comprising an inductance included in each of the anodecircuits.

2. In combination with an electric power converting apparatus includinga plurality of vapor electric discharge valves each provided with ananode, a cathode, and a control grid, the grid and anode of each of saidvalves forming a substantial internal capacity, means for rapidly andperiodically transferring the current between said electric valves, theanode circuits of said valves being subject to periodic potential wavesof steep wave front dependent upon the periodic transfer of currentbetween said valves, a control circuit for said grids, and means] forsubstantially eliminating the effect on said control circuit of saidperiodic potentials in the anode circuits comprising an inductance oflow distributed series capacity included in each of said anode circuits.

3. In combination with an electric power converting apparatus includinga plurality of vapor electric discharge valves each provided with ananode, a cathode, and. a control grid, the grid and anode of each ofsaid valves forming a substantial internal capacity, means for rapidlytransferring the current between said electric valves, the anodecircuits of said valves being subject to periodic potential waves ofsteep wave front dependent upon the periodic transfer of current betweensaid valves, a control circuit for said grid for successivelymaintaining said valves non-conductive for predetermined intervals, andmeans for substantially eliminating the effect of said periodicpotentials in the anode circuits tending to render said valvesconductive against the action of said control circuit comprising aninductance in each of the anode circuits immediately adjacent the anodeof its respective valve.

4. In combination, a direct current supply circuit, an alternatingcurrent load circuit, an inductive winding and a plurality of vaporelectric valves interconnecting said circuits, each of said valves beingprovided with an anode, a cathode, and a control grid, the grid andanode of each of said valves forming a substantial internal capacity, acapacitor for commutating the current between said valves and therebysubjecting said anodes to periodic potentials of steep wave form, acontrol circuit for said grids for successively maintaining said valvesnonconductive for predetermined intervals, and means for substantiallyeliminating the effect of said periodic potentials in the anode circuitstending to render said valves conductive against the action of saidcontrol circuit comprising an inductance device included in the anodecircuit of each of said valves.

ORRIN W. LIVINGSTON.

